I'm new to cloud storage and I'm wondering how a mail-client that is running on, say a rackspace server running RedHat, stores emails locally on it's machine so we can back emails up. I am trying to find the best mail client that can fit these requirements:

Backup of these email accounts is as follows:
1) Each email account must be backed up individually on a daily basis
2) We must be able to reconstruct our mail account as it looked on some past date.
3) Emails among these email accounts cannot be mingled. In other words, if we are asked to restore our "support" mail account, we should not be including emails from our "reports" mail account.

This client will:
1) Sit on a secured server accessible by us
2) Connect to each target email account over IMAP, securely
3) Synch down all emails for each account and retain emails on the file system. Each mail account must be stored in a location that is distinct from the locations of other accounts we are backing up (per regulatory requirement above). Thunderbird, for example, supports the notion of Profiles.
4) Each Profile maintains its own distinct folder structure on the fs. Each Profile is then responsible for pulling data for only of the mail accounts.
5) Mail stored within these profiles should be encrypted with the security key in our possession. This requirement will most likely be satisfied by the server infrastructure we select, ie the cloud servers at amazon will run on an encrypted file system.

If anyone has any ideas for an ideal replacement to thunderbird that would be of huge help.